Leukocytosis and thrombocytosis are associated with increased atherothrombotic risk in the general population. LNK is a scaffolding protein that in mice inhibits thrombopoietin signaling in hematopoietic stem cells (HSCs) and megakaryocyte progenitors. A LNK variant (R262W) has been associated with leukocyte, platelet counts and CHD in human GWAS. Using human cord blood we discovered an association of the TT risk SNP (R262W) of LNK with expansion of HSCs, increased TPO signaling and enhanced megakaryopoiesis, indicating reduced LNK function and increased MPL signaling. To assess the role of reduced LNK function in atherothrombosis, we transplanted WT or Lnk-/- bone marrow (BM) into irradiated WT or Ldlr-/- mice and fed recipient mice chow or Western diets (WD), respectively. While the Lnk-/- BM recipients showed similarly increased platelet counts with both diets, platelet activation, leukocyte counts, formation of platelet/leukocyte aggregates (PLA), atherosclerosis and thrombosis were markedly increased by hematopoietic Lnk deficiency combined with hypercholesterolemia. Thus we hypothesize that in the setting of hypercholesterolemia, genetic variants that cause activation of TPO signaling predispose to atherothrombosis via leukocytosis and platelet activation. Aim 1 will determine the mechanisms of platelet activation and leukocytosis in mice with hematopoietic Lnk deficiency and hypercholesterolemia. Aim 2 will assess mechanisms of accelerated atherosclerosis and thrombosis in these mice and the relative contributions of increased leukocytes, activated platelets, platelet-leukocyte aggregates and neutrophil-derived NETs to these processes. Therapeutic interventions with JAK2 or ERK inhibitors or rHDL infusions will be assessed. Aim 3 will employ human cord blood stem cells in a humanized mouse models to assess the impact of the TT risk SNP on hematopoietic functions in human cells.